Due to technology limitations, early radar development had to rely on the HF/VHF/UHF bands. The ever-increasing demand for frequency bands for radio use has forced some radar applications out of the lower bands.
However, the VHF/UHF bands remain attractive even today where microwave technology makes it feasible to operate in high frequency spectra.
Airborne Low Frequency Synthetic Aperture Radar operating in the VHF and UHF bands is becoming an important emerging technology for wide area surveillance and target detection in foliage. A VHF synthetic aperture radar system denoted CARABAS™ (trade mark?) (Coherent All Radio Band Sensing) SAR system has been described in U.S. Pat. Nos. 4,965,582 and 4,866,446. An ultra wide band coherent radar system has been disclosed U.S. Pat. No. 6,072,420.
To obtain sufficient resolution, viz. a few meters or less, the radar-operating band must extend over several tens of Megahertz, i.e. radar reception must be wide band and occur across frequency bands allocated for radio and TV broadcasting.
The development of airborne multi-octave foliage penetration radars in the VHF and UHF band renews the discussion on how radar and radio can co-exist in the same frequency band. The cohabitation issue is particularly pronounced with television broadcast due to its need for a large bandwidth.
Consider a VHF radar operating in an area of broadcast transmission. Given that radar transmit power is significantly below that of the commercial broadcast station, the operating range of television broadcast stations and the radar emission is of the same order of magnitude and finally that the television subscribers have directional antennas pointing at the broadcast station, the absolute majority of subscribers will receive a television signal, which dominates over the interference from the radar transmitter with respect to power level. The radar receiver on the other hand will sense a one-way propagation television interference signal, dominating strongly over the backscattered radar signal.
The fidelity of analogue television colour reception calls for a very good signal to noise ratio, enabling typically phase representation to within 0.5 deg. This would require 40 dB SNR. Assuming PG=1 MW (given directivity of television transmitter and receiver antenna), λ=5 m and r=100 km one finds
            P              TV        →        TV              =                  PG        ⁢                              λ            2                                                              (                                  4                  ⁢                  π                                )                            2                        ⁢                          r              2                                          =                        -          18                ⁢                                  ⁢        dB        ⁢                                  ⁢        m                        B      TV        =          5      ⁢                          ⁢      MHz      Exterior noise at VHF may be 40 dB above thermal noise, viz forPe=FekTB=−70 dBmwe find that 40 dB SNR gives a 10 dB margin to exterior additive noise. The cited figures seem characteristic of good receiving conditions and thus the expectation that possible interference from radar should not affect TV reception quality.
Now, consider interference caused by 20-90 MHz VHF SAR operating on an aircraft at some typical altitude of 10 km. Say that peak radar transmit power is 1 kW. A linear FM transmit waveform thus radiates 1 kW also in the TV band. With a 0 dB transmit gain (typical omni-directional low frequency SAR antenna) and a 10 dB receiver antenna gain we get PG=10 kW and one has
      P          Radar      →      TV        =            PG      ⁢                        λ          2                                                    (                              4                ⁢                π                            )                        2                    ⁢                      r            2                                <                  -        18            ⁢                          ⁢      dB      ⁢                          ⁢      m      
Hence, as a worst case, the radar interference and the useful TV signal are of the same order of magnitude. Using a more advanced radar waveform, in which the TV band transmission is present over the entire length of the radar transmit signal, the peak power in the TV band can be reduced by 10 dB. Still compared to the required TV SNR ratio, the interference level remains high.
Another important aspect concerning cohabitation is the radar duty cycle. The peak power figure presumes a relatively large duty cycle—higher than 10%. The relative time under which the TV reception is interfered is thus higher than 1%, depending on the waveform and to some extent the interference level as a trade-off parameter.
TV broadcast antennas are vertically extended in order to depress the transmit signal towards its terrestrial users. For this reason and for the absence of gain in the radar antenna, one may set PG=10 kW for radar reception of a single TV station. As there may be a number of stations active in the spectrum, as a worst case we assume PG=100 kW and get instead of 0
      P          TV      →      Radar        =            PG      ⁢                        λ          2                                                    (                              4                ⁢                π                            )                        2                    ⁢                      r            2                                =                  -        28            ⁢                          ⁢      dB      ⁢                          ⁢      m      
As mentioned exterior noise at VHF may be 40 dB above thermal noise, viz. for Bradar=50 MHzPe=FekTB=−60 dBm
Given the elevated exterior noise temperature, radar transmit power has to be increased to compensate for this noise. The sited figures for radar transmit power are calculated on this presumption. Giving rise to interference levels for television reception of the same order as the actual television signal they can hardly be increased. Hence the 30 dB noise figure of television interference has to be mitigated by other means.
The suppression depth of the mitigation step must consequently be 30 dB. There are several basic mitigation techniques                1. Range spectrum band-pass filtering        2. Doppler spectrum filtering        3. Cancellation        
The first method implemented by narrow-band notching techniques that can be analogue pre-reception has been explored in connection with the VHF radar work done in Sweden. This option is only feasible if at least the partial band 20-50 MHz is free from TV-interference (?). Removing a significant part of the radar spectrum leads to production of sidelobes, which severely degrades radar performance.
The second method is normally associated with the possibility of removing carrier waves, letting the radar operate coherently in slow time by digital post-reception carrier cancellation techniques. Removal of carrier waves is considered inefficient since in the matched filtering performed by the SAR-processing, the carrier signal combines with its modulation leading to an overall increase in the noise floor.
The third method of cancellation presumes that, the TV signal by some means can be identified and subsequently subtracted from the combined radar/television signal. Active noise cancellation is known in the audio field in which ambient noise is recorded and a mirror signals is processed, amplified generated by means of a loudspeaker. The mirror signal is generated in a timely and accurate fashion so as to cancel the noise at a given point. Since the TV signal has many degrees of freedom, and the required cancellation depth is significant, it is doubtful if such a cancellation can be successful in practice. Even the much simpler case of cancelling a pure carrier is by experience a delicate matter.